In order to significantly reduce nitrogen oxide (NO.sub.x) emissions from gas turbine engines, multiple combustion chambers are typically employed within the burner section of an engine. Fuel is introduced to each combustion chamber through one or more respective nozzles, and the sets of nozzles are typically referred to as pilot nozzles and main nozzles. At low fuel flow rates, fuel is fed only to one or more of the pilot nozzles, whereas at higher fuel flow rates, fuel is fed or staged to one or more main nozzles in addition to the pilot nozzles.
One known system for metering fuel to two sets of nozzles is shown, for example, in U.S. Pat. No. 4,949,538 to Iasillo et al. In this system, a conventional flow metering unit (FMU) meters the total fuel flow required by the engine, and a downstream staging valve divides the total fuel flow, directing a portion to the pilot nozzles, and the remainder to the main nozzles. When a main nozzle is initially activated, the fuel flow to the corresponding pilot nozzle(s) is temporarily diverted to the main nozzle flow path until the main nozzle flow path, typically including a nozzle manifold, is filled with fuel. As a result, although the fuel system is attempting to increase the flow of fuel to the combustion chambers, the overall flow of fuel to the combustion chambers is temporarily reduced in an aircraft application, this condition is not acceptable.
It is an object of the present invention to overcome the drawbacks and disadvantages of prior art fuel staging systems.